Stainless steel belts which are used in the manufacture of optically clear polymeric sheet, such as acrylic sheet, must be flat and must have an optically polished finish on the casting side of the belt. When the steel, from which the belt is made, is manufactured at a steel mill, it is generally softer and thicker than is desired. In order to obtain the desired thickness and to increase the hardness of the steel to give it sufficient strength so that it will not stretch when used with a tensioning system, such as cylinders or other tensioning devices, over which the belt revolves and is kept substantially flat, the steel is cold rolled. The cold rolling operation results in the introduction of surface stresses on both surfaces. When additional stresses are introduced onto one surface of the belt, for example the top, the belt moves to restore the equilibrium. Thus, deformations in the surface of the belt are compensated for, to reestablish the stress equilibrium, by deformations on the back of the belt. Additionally, deformations on the back of the belt will result in deformations on the mirrored surface of the belt. These deformations will appear in the material being cast on the mirrored surface of the belt.
When the belt is in use, the cylinders over which the belt rotates may be 100 feet or more apart and will be suitably tensed so that the mirrored surface of the belt will be substantially flat. Because the distance between the cylinders may be 100 feet or more, it is necessary that supporting means, such as steel idler rolls, extend at spaced intervals across the width of the belt. These supporting means, such as steel idler rolls, are generally not driven and serve to support the belt. However, such support actually takes place at points of contact between the supports and the belt. These point contacts, because of friction or compressive effects, eventually scratch, planish, or abrade the underside of the belt at the contact points. This abrading, planishing or scratching disturbs the stress equilibrium in a local area so that the change of stress on a local area on the back of the belt results in a shape change on the corresponding local area on the mirrored surface of the belt in order to restore the stress equilibrium. This results in a deformation of the mirrored surface and is translated onto the surface of the material which is being cast on the mirrored surface.
When the deformation on the mirrored surface becomes substantially severe so that articles cast thereon are not commercially acceptable, the belt is removed and replaced with a new belt and the old belt is generally sold for its scrap value. This results in the loss of production time and the expenditure of considerable sums of money for a new belt.
It is an object of this invention therefore to provide a process for refurbishing a distorted steel belt used for casting polymeric materials such as acrylic sheet.
Another object of this invention is to refurbish a new belt so that the new belt has less distortion than when received from the manufacturer.
Other objects and advantages will become apparent from the following more complete description and claims.